Tips when playing Sejuani:

Melee allies can stack Frost for your Permafrost, keep an eye on who they are attacking.

Save Glacial Prison for encounters where you have teammates present to take advantage of it.

The story of Sejuani:

Sejuani is the brutal, unforgiving Iceborn warmother of the Winter’s Claw, one of the most feared tribes of the Freljord. Her people’s survival is a constant, desperate battle against the elements, forcing them to raid Noxians, Demacians, and Avarosans alike to survive the harsh winters. Sejuani herself spearheads the most dangerous of these attacks from the saddle of her drüvask boar Bristle, using her True Ice flail to freeze and shatter her enemies.